Conventionally, there has been known a charge-discharge system, which can charge an electric storage device mounted on a vehicle with (by) a power source (supply) external to the vehicle, and which can supply an electric power from the electric storage device mounted on the vehicle to an electric load external to the vehicle (including an electric storage device external to the vehicle, and the like). Supplying the electric power from the electric storage device mounted on the vehicle to the electric load external to the vehicle means discharging in terms of (or for) the electric storage device mounted on the vehicle. Accordingly, in the present specification, supplying the electric power from the electric storage device mounted on the vehicle to the electric load external to the vehicle may also be expressed as “discharging to the electric load external to the vehicle.” Further, the electric storage device mounted on the vehicle may also be referred to as a “vehicle (on-board) electric storage device”, and the electric storage device external to (outside of) the vehicle may also be referred to as an “external (off-board) electric storage device.” Furthermore, the power source (supply) external to (outside of) the vehicle may also be simply referred to as an “external power source”, and the electric load external to (outside of) the vehicle” may also be simply referred to as an “external electric load.” The external electric storage device can be the external electric load as well as the external power source.
Meanwhile, as a standard for an electric vehicle whose vehicle electric storage device can be charged from a home (external power source), “SAE Electric Vehicle Conductive Charge Coupler (refer to Document No. 1 below)” has been adopted in the United States of America”, and a “general requirement for electric vehicle conductive charge system” has been adopted in Japan (refer to Document No. 2 below).
Document No. 1; United States of America Standard SAEJ1722, SAE international, November, 2001
Document No. 2; General requirements of conductive charge system for an electric vehicle (Japan Electric Vehicle Association (JEVA) standard), Mar. 29, 2001
In those standards, for example, a standard regarding “control pilot” is defined. The control pilot is a control line, and is defined as follows.                Equipment ground (grounding conductor)—A conductor used to connect the non-current carrying metal parts of the EV supply equipment to the system grounding conductor, the grounding electrode conductor, or both at the service equipment.        Control pilot—The primary control conductor that is connected to the equipment ground through control circuitry on the vehicle and performs certain functions.        Electric Vehicle Supply Equipment (EVSE)—The conductors, including the ungrounded, grounded, and equipment grounding conductors, the electric vehicle connectors, attachment plugs, and all other fittings, devices, power outlets, or apparatuses installed specifically for the purpose of delivering energy from the premises wiring to the electric vehicle.        
A signal transmitted through the control line (control pilot) is referred to as a control pilot signal, a CPLT signal, or simply a “CPLT.” The control pilot signal is used by the control circuit on the vehicle (on board control apparatus) to perform the following functions:                a. verifies that the vehicle is present and connected;        b. permits energization/de-energization of the supply;        c. transmits supply equipment current rating to the vehicle;        d. monitors the presence of the equipment ground; and so on.        
On the other hand, in the charge-discharge system, the vehicle control unit controls “charging (charge of) the vehicle electric storage device using the electric power supplied from the external power supply” and “discharging (discharge) to the external electric load form the vehicle electric storage device.” Accordingly, the control unit of the vehicle needs to recognize/determine whether the charge of the vehicle electric storage device should be performed using the electric power supplied from the external power source (that is, request for charge is being generated) or the discharge from the vehicle electric storage device to the external electric load should be performed (that is, request for discharge is being generated).
One of known arts uses a charging cable, and a discharging cable different from the charging cable, and differentiates the control pilot signal for the charging cable from the control pilot signal for the discharging cable. Further, when the power cable is connected with the vehicle, the vehicle control circuit is configured so as to determine based on the control pilot signal whether the connected power cable is the charging cable or the discharging cable, and determine based on the determination whether the request for charge is occurring or the request for discharge is occurring (refer to Japanese Patent Application Laid-Open (kokai) No. 2010-35277, paragraph 0072 etc.).